1. Field of the Invention
The invention relates to the transmission of information in the form of an electromagnetic signal. In particular, the invention concerns the allocation of carrier frequencies for transmission systems using multi-carrier type modulation and the process for reducing the ratio of peak amplitude to mean amplitude usually required in transmission systems using multi-carrier type modulation.
2. Description of the Related Art
An information transmission system generally sends symbols serially, where each symbol can be a sequence of binary data. Consequentially, the frequency band required to send the symbols must be larger than the inverse of the length of a symbol. When the symbol transmission rate becomes to high, the channel must have identical amplitude and phase characteristics over the entire space of the frequencies which constitute the passband. Any distortions will give rise to interference between symbols, which must be negated with an equalizer.
One method of avoiding this problem is to distribute the signal, formed by a stream of symbols, over a plurality of parallel carriers, which are individually modulated at a low transmission rate. Because the transmission rate is low for each carrier, the passband required is smaller and therefore the frequency and phase characteristics will more likely be identical for all the frequencies constituting this band.
This technique is generally known as frequency division multiplex, and is used to select carriers so as to avoid interference. One particular case is Orthogonal Frequency Division Multiplex, or OFDM, in which the spacing between two adjacent subcarriers (the closest subcarriers in terms of frequency) corresponds to the inverse of the length of a symbol sent.
As a result of flaws in the transmission channel, a transmitted symbol can contain errors on reception, and, if the errors are detected, retransmission may be required.
To improve this situation, it is possible to transmit a series of blocks of symbols, where each of these blocks is a discrete Fourier transform or inverse Fourier transform of a corresponding block of information symbols.
The advantage of this technique is that all of the symbols received will be affected by only a small evaluation error in the event of a transmission channel problem. If the procedure were not applied, a single symbol would be affected by a large evaluation error, leading to erroneous detection. It is an object of the invention to correctly evaluate all symbols using the fast Fourier transforms demodulation technique.
This technique is also a particular method of OFDM. To appreciate the similarity of this technique to OFDM, reference can be made to chapter 15 of “Modem Quadrature Amplitude Modulation Principles and Applications for Fixed and Wireless Channels,” by W. T. Webb and L. Hanzo.
OFDM operates as follows. Initially, a complex vector comprising n components for transmission is transformed with an inverse Fast Fourier Transform (IFFT). The complex vector can be, for example, complex numbers forming part of a whole creating an alphabet, or code adapted to correspond to the different sequences of data for transmission. The transformation by IFFT may be by a matrix product of an inverse Fast Fourier transform matrix, or “Fourier Matrix”, into n rows and n columns by the vector of the n data elements for transmission. The alphabet is generally that of the phase and amplitude modulations.
A vector of n complex numbers, known as a “transformed vector, is generated from this matrix product. The transformed vector forms a succession of numbers the amplitudes of which are transmitted successively by the device. This series of amplitudes, referred to as a baseband OFDM symbol corresponds to a sequence of n data elements for transmission.
This signal can itself modulate a carrier of a higher frequency to be able to be transmitted in a transposed band, according to a conventional technique.
Baseband reception or demodulation occurs by multiplying the received transformed vector with the matrix of the direct Fast Fourier transform, or the matrix of the inverse transform if the direct Fast Fourier transform was used on transmission). The received vector is a counterpart of the vector obtained from OFDM transmission, although it has been subjected to interference, noise addition or partial fading.
OFDM demodulation does not, therefore, restore the initial components of the complex vector associated with the sequence of data for transmission, but rather it approximates the components. The information is restored after a decision making process which consists of measuring the distance of each component calculated after reception at each point of the encoding alphabet used for transmission, and of assimilating the component calculated after reception to the point of the alphabet that corresponds to the shortest distance.
Instead of having most of the data received perfectly and a few data elements completely lost, as in conventional series transmission, the transmission errors are, in fact, distributed over all of the points, which ensures that it is almost always possible to reconstitute the initial information in its entirety.
This conventional mode of transmission using OFDM does, however, have a major drawback. Through the effect of the matrix product, the discrete Fast Fourier transform creates a linear combination of the n symbols for transmission and a number of critical complex vectors, associated with critical sequences of data. This combination can result, after the Fourier transform, in transformed vectors wherein the succession of amplitudes of the components have local maximum values corresponding to signal peaks that are substantial, in relation to the mean value for the amplitudes of the components of the transformed vector.
The peak amplitude to mean amplitude ratio of the transformed vectors corresponding to these critical sequences or critical complex vectors is thus very high.
Such critical sequences may cause difficulties for the downstream devices as, in practice, amplifiers and modulators may lack the fidelity to process swift amplitude variations. As a result clipping, which is the non-transmission of the signal peaks, may occur, resulting in the loss of corresponding information. Furthermore, harmonic distortion, one of the major problems of transmission systems, may be introduced, and may be impossible to negate.
Theoretically, maximum amplitude is calculated to be a direct function of the length of the sequence of symbols for transmission.
It is thus highly desirable to reduce this maximum amplitude so as to use the full dynamic properties of the amplifiers without causing clipping or distortion. Several solutions aimed at alleviating this problem of peaks are known. One of these techniques is to exclude sequences of symbols creating maximum peak-to-mean amplitude ratio values of the OFDM symbol. This is achieved by encoding redundancies, resulting in a reduction in the transmission rate of useful symbols. One example of implementation of this solution is described in U.S. Pat. No. 5,636,247.
Another solution is to calculate the inverse Fourier transform for the sequences of symbols to be transmitted, and then to measure the peak-to-mean ratios for the transformed vectors thus obtained, and, by looping, to change the phases of the components of the critical complex vectors corresponding to the peaks. Measurement of these peaks involves calculating another discrete Fourier transform. A technique of this kind is disclosed in U.S. Pat. No. 5,610,908. A third solution is to change the coefficients of the Fourier matrices (inverse and direct) so as to avoid or limit the occurrence of these peaks. This process induces a slight deterioration in the bit error rate. By way of example, one solution of this type has been proposed by patent application FR 98.13261.
All these currently implemented solutions have the drawback either of adversely affecting the bit rate of the transmission, impairing the quality of the transmission, or of being complicated.
A conventional multicarrier transmission system of standard type (see FIG. 1) has a data source 910 a serial-to-parallel converter 920 connected to a stream of subcarriers, and a multi-carrier modulator 950 which transmits the data to an RF transmitter 960. In a standard system of this type, the data are distributed sequentially over the different subcarriers. For example, for a system using eight subcarriers, the data bearing the numbers 0, 8, 16, 24 will be transmitted over the subcarrier 930 of frequency ω1, the data bearing the numbers 1, 9, 17, 25 will be transmitted over the subcarrier frequency ω2 etc.
In a conventional device not according to the invention, this stream of the “serial” type is converted into a “parallel” stream by the serial-to-parallel converter 920, so as to reduce the transmission rate of the modulating signals. This parallel stream is then sent to the multi-carrier modulator 950, which effects the modulation necessary for the transmission over the chosen transmission channel.
In the example presented, the serial stream is transformed into a parallel stream in eight bits. In this case, if the transmission rate of the binary source is D, the rate of each stream at the output of the serial-to-parallel converter 920 will therefore be D/8.
Each of these stream then modulates a subcarrier by virtue of the subcarriers 930 to 937. The modulation can be of different types: phase, amplitude or frequency modulation, according to conventional techniques.
An adder 940 next adds all the modulated subcarriers so as to obtain the global signal S(t), which is then transmitted to the RF transmitter 960.
It is significant to note that the binary data X01, X1, X2, . . . X7 issuing from the serial-to-parallel converter 920 and used for modulating the subcarriers, can consist of several bits. They will then more generally be referred to as “symbols”. In this case the modulations employed can be complex (for example according to types known to persons skilled in the art as QPSK, 8PSK, 16QAM, 64QAM etc) in order to improve the spectral efficiency.
These elements constitute a conventional multicarrier device, known to persons skilled in the art. It will therefore not be detailed any further in the present description.
The majority of the transmission channels, or “radio” channels used have transmission characteristics, such as attenuation, noise, or phase displacement, which vary depending on the carrier frequency used. Certain channels have characteristics which vary over time, because of “multipath” effects, such as the presence of elements entering the channel.
FIG. 2 depicts an example of a symbolic representation of the transmission quality, quantified by a signal to noise ration, or “SNR”, on each of the subcarriers in the case of eight subcarriers, at two different times, Time t1 and Time t2. The transmission characteristics for each frequency varying with time, it is found in the example that the data item X6 is correctly transmitted at time t1, but may be erroneous at time t2.
The concept of efficiency of such a multicarrier transmission is then related to the resolution of the following problem: with what power P must transmission be carried out in order to ensure the transmission of a certain output of data D with a quality Q in a given physical transmission channel?
This efficiency can be defined as the ratio                (transmission rate x quality)/emitted power        
The solution generally adopted for this problem of transmission efficiency is a compromise between on the one hand the energy emitted during transmission over the transmission channel and on the other hand the acceptable error rate for the transmitted data.
The operating principle of the majority of existing devices is to increase the transmission power in order to counteract the degradation of the transmission channel and to transmit all the data with guarantee of an error rate below a predetermined threshold.
Several techniques have been disclosed for improving the efficiency of transmission.
These techniques are based on a different coding for the data considered to be the most significant, before sending over the transmission channel.
A technique disclosed in U.S. Pat. No. 5,425,050 introduces a concept of pyramidal coding, in which two classes of data requiring two different transmission quality levels are created.
U.S. Pat. No. 5,467,132 describes a method for coding the data differently according to their significance.
Other techniques are based on a dynamic estimation of the transmission quality on each subcarrier, and on a modification of number of bits per symbol transmitted in order to take account of this variation in transmission quality. U.S. Pat. No. 5,479,447 describes one example of this technique.
In summary, the conventional solutions to this problem of multicarrier transmission efficiency are:                increasing the transmission power so as always to transmit with a sufficient signal/noise level,        testing the transmission channel and eliminating the subcarriers most interfered with,        adding redundancy to the data by coding,        modifying the number of bits per symbol for the subcarriers interfered with.        
All these solutions result in an increase in the emitted energy for transmitting the same data stream with a constant quality.